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Have a good backup of everything before starting in case there are problems. You won't be able to use Migration Assistant to move things from Yosemite to Mountain Lion because it will see that you're "going backwards" and won't allow you to. Now your Mac system will have both a Mountain Lion install as well as the Yosemite install. run the mountain lion installer and install on the new disk/partition you created in step 1 - it will also create another recovery partition from this same part of the disk

Reboot Yosemite and then boot from the USB thumb drive by holding down the option keyĤ. Create an external bootable USB thumb drive with the Mountain Lion OS on it (do a search on the web for this - for example see tall_drive.htmlģ. have an external disk or create another partition on an internal drive to use for the Mountain Lion install (you might have to shrink an existing partition to make room for another)Ģ. run the mountain lion installer and install on the new disk/partition you created in step 1 - it will also create another recovery partition from this same part of the disk. I think the problem is that you can't install an older OS X version onto a system running a newer OS X version.